Russia will treat it as an existential war.
It's like Russia establishing missile bases on Cuba.
Not exactly. NATO nations already border Russia.
Russia's motivation/justification for expanding its rule, including the invasion of Ukraine, can be understood by Putin's (and many Russians) worldview that emphasizes the historical unity of Eastern Slavs—Russians, Ukrainians, and Belarusians—tracing their origins back to the middle ages.
Putin will expand Russian control over eastern nations once part of the Soviet Union - if allowed.
“The difficulty in understanding the Russian is that we do not take cognizance of the fact that he is not a European, but an Asiatic, and therefore thinks deviously. We can no more understand a Russian than a Chinaman or a Japanese, and from what I have seen of them, I have no particular desire to understand them, except to ascertain how much lead or iron it takes to kill them. In addition to his other Asiatic characteristics, the Russian has no regard for human life and is an all out son of bitch, barbarian, and chronic drunk.” --Statement by Patton on 8 August 1945, as quoted in “General Patton : A Soldier’s Life”.